1. Field of the Invention
The present invention relates to an information playback apparatus, a signal processing apparatus, an information playback method, and an optical recording medium for use in reproducing information on an optical recording medium, and more particularly to an optical recording medium having short intervals of tracks, and an information playback apparatus, a signal processing apparatus, and an information playback method for use in reproducing information on the optical recording medium.
2. Description of the Related Art
Recently, optical discs represented by CD and DVD are used practically. Specifically, CD-R (CD-Recordable) which can record digital data only once and CD-RW (CD-ReWritable) which can rewrite digital data a plurality of times, as well as CD-DA (CD-Digital Audio) that is a recording medium for playback-only, have been put into practical use.
At a time of recording and reproducing in an optical disc, it is necessary to rotate the optical disc at a predetermined speed. A playback-only recording medium can determine a predetermined speed by synchronizing the rotating speed with the reproducing frequency of digital data at playback. Contrarily, a recordable recording medium such as CD-R and CD-RW cannot control the rotating speed in the above way, because digital data is not initially recorded on the tracks. Therefore, in the recordable medium, tracks are wobbled in correspondence with address information, thereby controlling a rotating speed based on wobble signals read from the tracks and recognizing the addresses of the tracks.
As a recording method of the address information by wobbling, which is now in practical use, a method of wobbling a track according to an FM-modulated wobble signal is known. A method of wobbling tracks according to the address information by phase-modulation of a wobble signal is disclosed in U.S. Pat. No. 5,999,504 (corresponding to Japanese Patent Publication Laid-Open No. H10-69646).
On the other hand, further improvement in recording density is desired to an optical disc. When increasing the recording density of an optical disc, a space between spiral tracks (the space in the radius direction of an optical disk) cannot help getting narrower. Therefore, it becomes difficult to thoroughly focus a laser spot on a predetermined track, and there is a problem of generating crosstalk from adjacent tracks.
In a conventional apparatus, crosstalk of an RF signal is eliminated on the basis of an error between an actual signal from a photo-detector and an ideal signal.
However, when recording the address data by phase modulation of wobble, the wobble signal waveform becomes a sine wave. This is why a carrier signal, which carries address data is recorded by wobbling the group in an analog way. Generally, it is difficult to detect the error (crosstalk amount) from this analog signal waveform. A signal before actual demodulation has a random noise and it becomes very noisy. Therefore, it is practically impossible to detect an error.